Add string << operators for the other streams as well, and removes

Add string << operators for the other streams as well, and removes
c_str() all places where it is not needed anymore.
(I also add config.h to several files that was missing it)
+ added s string constructor to math_symbolinset

I also change some "while(" to "while ("

mmm ... yes ... I add a "zero()" function to LyXLength... an "empty()"
function did not seem appropriate. changed a couple of places that
checked on length.value() != 0.0.

git-svn-id: svn://svn.lyx.org/lyx/lyx-devel/trunk@3149 a592a061-630c-0410-9148-cb99ea01b6c8